1. Field of the Invention
The present invention relates to an exposure apparatus for performing the exposure with an image of a pattern projected by a projection optical system in a state in which a space on a side of an image plane of the projection optical system is locally filled with a liquid. The present invention also relates to a method for producing a device by using the exposure apparatus.
2. Description of the Related Art
Semiconductor devices and liquid crystal display devices are produced with the so-called photolithography technique in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus, which is used in the photolithography step, includes a mask stage for supporting the mask and a substrate stage for supporting the substrate. The pattern on the mask is transferred onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. In recent years, it is demanded to realize the higher resolution of the projection optical system in order to respond to the further advance of the higher integration of the device pattern. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Therefore, the exposure wavelength, which is used for the exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used in some situations. When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ is too narrowed, it is difficult to match the substrate surface with respect to the image plane of the projection optical system. It is feared that the margin is insufficient during the exposure operation. Accordingly, the liquid immersion method has been suggested, which is disclosed, for example, in International Publication No. 99/49504 as a method for substantially shortening the exposure wavelength and widening the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent to utilize the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases) so that the resolution is improved and the depth of focus is magnified about n times.
The conventional technique as described above involves the following problem. The conventional technique is constructed such that the space, which is disposed between the substrate (wafer) and the lower surface on the side of the image plane of the projection optical system, is locally filled with the liquid. When a shot area, which is positioned in the vicinity of the center of the substrate, is subjected to the exposure, the liquid does not outflow to the outside of the substrate. However, as schematically shown in FIG. 14, when it is intended to expose an edge area E of the substrate P by moving the circumferential area (edge area) E of the substrate P to the projection area 100 of the projection optical system, the liquid outflows to the outside of the substrate P. If the outflowed liquid is left to stand, the environment (for example, humidity), in which the substrate P is placed, is consequently varied to cause, for example, the change of the refractive index on the optical path of the interferometer for measuring the position information about the substrate stage for holding the substrate P and/or on the optical path of the detecting light beam of each of various optical detecting devices. As a result, it is feared that any desired pattern transfer accuracy cannot be obtained. Further, an inconvenience also arises, for example, such that any rust appears due to the outflowed fluid on mechanical parts or the like arranged around the substrate stage for supporting the substrate P. It is also conceived that the liquid is prevented from any outflow by omitting the exposure for the edge area E of the substrate P. However, if the pattern is not formed by applying the exposure process to the edge area E as well, another problem arises as follows. That is, the substrate P as the wafer abuts against the polishing surface of the CMP apparatus in an unbalanced manner, for example, in the CMP (chemical mechanical polishing) process as the downstream step, and it is impossible to perform any satisfactory polishing. Further, if the outflowed liquid makes inflow into the tube of the vacuum system (suction system), it is also feared that the vacuum pump, which serves as the vacuum source, may be damaged or broken down.